refactor: combine typst-preview and tinymist extension code

[Myriad-Dreamin]

Previous PRs:

• 337

• 332

• 323

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Start to merge extension code, and we make some "compatible" branch to override logic. Since this PR, you don't have to install two extensions and all preview configurations and commands are moved to myriaddreamin.tinymist.

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The only two non-trivial changes of mgt19937.typst-preview:

• override getCliPath by tinymist.getServer

  export async function getPreviewCliPath(extensionPath?: string): Promise<string> {
    if (tinymistServerConfig) {
        return getServer(tinymistServerConfig!);
    }
    ...}

• remove typst-guy. since we will have a unique status indicator in end.

  let guy = "$(typst-guy)";
  export async function setIsTinymist(config: Record<string, any>) {
    guy = "$(sync)";
  }